1. Field of the Invention
The present invention relates to a system and method for controlling an engine idling speed for an internal combustion engine. The present invention specifically relates to the system and method therefor which can control the engine idling speed to an optimum value stably from a start at a low ambient temperature to a warm-up state.
2. Background of the Art
A Japanese Utility Model Application First (Unexamined) Publication No. sho 57-58181 published on Apr. 6, 1982 exemplifies a previously proposed engine idling speed controlling system in which an electromagnetic valve for controlling an airflow quantity in a passage bypassing a throttle valve of the engine is installed so that the engine idling speed is fed back to coincide with a target engine idling speed.
In addition, a Japanese Utility Model Application First (Unexamined) Publication sho 62-57750 published on Apr. 10, 1987 exemplifies another system for controlling the engine idling speed with a change in an engine cooling water temperature taken into account.
In the latter system disclosed in the above-identified Japanese Utility Model Application Publication, a water jacket is installed as a cooling water chamber in a periphery of a coil portion of the electromagnetic valve and a magnetic force generated by the coil portion is varied according to a temperature of the cooling water so that a change characteristic of the airflow quantity in the bypass passage is varied by means of the electromagnetic valve for each temperature of the engine cooling water.
Therefore, a maximum air quantity derived by means of the electromagnetic valve at a temperature of the predetermined coil portion after warm-up of the engine can be reduced to a required airflow quantity. Therefore, even if the valve remains open due to inoperation of the valve caused by an overrun of a corresponding program or dirt contained in the intake air of the engine, the excessive increase in the engine idling speed can be prevented.
However, since the airflow quantity characteristic of the electromagnetic valve with respect to a voltage duty ratio of a signal applied to the coil portion of the electromagnetic valve is set so as to vary for each cooling water temperature, in the previously proposed latter engine idling speed controlling system, the following problem arises.
That is to say, in the feedback control of the idling speed, an actual engine speed calculated on the basis of a signal derived from a crank angle sensor or ignition coil is compared with a target engine speed determined depending on the cooling water temperature detected by a cooling water temperature sensor. If there is a difference in the comparison result, a control variable, i.e., control coefficient of the electromagnetic valve is corrected according to the difference so that the engine speed reaches the target engine speed. At this time, feedback control coefficients in the proportional-integration (P-I) feedback control mode, i.e., integration (I) and proportional (P) coefficients are defined. In details, the actual engine speed is compared with the target vehicle speed. When the actual engine speed is higher, the control integration coefficient is reduced by a predetermined integration quantity (I) with time. On the contrary, if the actual engine speed is smaller, the control integration coefficient is increased by the predetermined integration quantity (I). In addition, when the difference exceeds a predetermined value and the actual engine speed is smaller, the control proportional coefficient (P) in proportion to the difference is generated.
Since the feedback control coefficients are constant (integration and proportional coefficients) when the actual engine speed is controlled to match with the target engine speed determined according to the engine cooling water temperature, it takes a long time for the actual engine speed to reach the target engine speed. In addition, a hunting occurs after or before the target engine speed.